fix(api): add index on validatorindex column of the blocks_bls_change table

Author: peterbitfly

backend/pkg/commons/db/migrations/20250818072749_add_blocks_bls_change_validatorindex_index.sql

@@ -0,0 +1,9 @@
+-- +goose Up
+-- +goose StatementBegin
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_blocks_bls_change_validatorindex ON blocks_bls_change (validatorindex);
+-- +goose StatementEnd
+
+-- +goose Down
+-- +goose StatementBegin
+DROP INDEX CONCURRENTLY IF EXISTS  idx_blocks_bls_change_validatorindex;
+-- +goose StatementEnd